When a printing plate using a reprographic material, such as a PS plate, is to be prepared from a color original so that many prints will be made, halftone separation negatives (or positives) of normally four colors are prepared from the color original by means of a color scanner. Then, the negatives or positives are subjected to such steps as pasting-up, reversing and retouching, to form block copies of the respective colors. Proof prints are made using the block copies, and the state of a final finish is confirmed. Afterwards, printing is performed. In this case, checking is done for separation with the scanner in the step of preparing the halftone separation negatives or positives; for pasting-up, etc. in the step of preparing the separation positives; and for final finish in the proof printing step. For this checking work, color proofs are prepared at each step. Various photosensitive materials are utilized for color proof preparation. For instance, those for the silver halide-photographic process, those for electrophotography, and those using photopolymers are known. To produce color images, those containing pigments or dyes are used. Both the negative-positive process and the positive-positive process are available depending on the intended purpose. Typical examples of the methods for preparing color proofs include the following: First, a photosensitive layer of a photosensitive material having said photosensitive layer on a substrate is transferred onto a transfer paper by pressure and heat, whereafter the photosensitive layer is imagewise exposed and developed to form an image of a first color on the transfer paper. Then, a second photosensitive layer is transferred onto the transfer paper in the same manner, followed by imagewise exposure and development, thereby forming an image of a second color on the transfer paper. This procedure is carried out a total of 4 times for yellow, magenta, cyan and black, thereby forming a multicolor image. The transfer of the photosensitive layers onto the transfer paper can be performed by various methods, including those using only pressure, pressure and heat combined simultaneously, etc. An example of such method comprises heating one or both of a pair of pressurized rollers, and passing between the pair of rollers a photosensitive sheet having a photosensitive layer and a transfer paper superimposed on each other, thereby transferring the photosensitive layer onto the transfer paper. This method, however, poses problems in terms of the operating efficiency and the cost of preparing color proofs.
An image forming method, to which the photosensitive sheet of the present invention is applied, imparts imagewise exposure to said sheet, then carries out development, and transfers the resulting image to an image receiving sheet, thereby forming a color proof. Namely, this method comprises treating a photosensitive sheet with the below-mentioned steps, said photosensitive sheet having an image-forming colored photosensitive layer on a substrate via a peelable, heat-fusible resin layer.
(1) a step of exposing the photosensitive material with actinic radiation through an original in contact with the photosensitive material (this may be called contact exposure), and then developing the unexposed areas to form an image;
(2) a step of bringing the image-bearing surface into contact with a heat-fusion surface of a transfer material having a heat-fusible layer, heating and pressing both surfaces, then peeling the substrate from the heat-fusible resin layer, thereby transferring the colored image onto the transfer material together with the heat-fusible resin layer;
in which step (1) the photosensitive materials having the colored layers of different tones are made ready for use, the original is selected depending on each tone, and contact exposure and development are performed, whereby the images are formed, and
in which step (2) the colored image of a specified tone is transferred onto the transfer material; then
(3) a step of bringing the image bearing surface of the photosensitive material having formed thereon a colored image of a tone different from the above tone into contact with the colored image-transferred surface of the transfer material obtained in step (2), heating and pressing both surfaces, then peeling the substrate from the heat-fusible resin layer, thereby transferring the colored image onto the colored image-transferred surface of the transfer material together with the heat fusible resin layer; and then
repeating step (3) for a third or more colors.
In this case as well, the method of transferring the image onto the image receiving sheet involves the simultaneous execution of pressing and heating. Namely, one or both of a pair of pressure rollers is or are heated, and the image bearing sheet and the image receiving sheet are passed in a superimposed state between the rollers, thereby transferring the image. This method is different from the aforementioned method in which the photosensitive layer transferred onto the transfer paper is exposed to form an image. According to the former method, the formed color images are sequentially transferred onto the image receiving sheet one color at a time. Therefore, exposure for four colors can be performed at the same time, thus shortening the operating time. However, after the images are formed, they are transferred. Hence, misregister involving color distortion may occur easily even when the transfer of the images has been done accurately.
So far, the use of an ordinary transferring apparatus has required extremely high temperatures because of the short heating time. This has posed the problem that the sheets inserted between the rollers are partially elongated, causing the misregister or transfer unevennesses of the transferred images. The misregister is a serious problem since the transfer of images is normally performed 4 times.
To solve the problem of misregister, several methods have been proposed, including one in which the substrate, plastic film, is annealed to reduce its heat shrinkage and prevent dimensional changes (Japanese Laid-Open Patent Publication No. Sho. 63-305349). However, pressure is also applied during the actual transfer operation, and thus annealing alone cannot completely prevent dimensional changers of the plastic film. Hence, misregister cannot be eliminated. It is known that misregister is reduced if the width-wise positions of the plastic film formed are designated and only that amount of the film which corresponds to the same positions is used as the image bearing sheet. (For example, that amount of the film formed which corresponds to the width of 5,000 mm to 1,100 mm is slit, and four colors are allocated only at the same positions in the direction of flow). This will, however, considerably lower productivity during film formation or image bearing sheet production.
Even with the thus produced image bearing sheet, misregister cannot necessarily be corrected, if it is used in a small size, say, by taking a piece 508 mm.times.610 mm (width.times.length) from each of the left and right sides of the middle in the width direction of the film formed, and using the two portions in a mixed manner. Hence, only either the right or left portion must continue to be used, making operating efficiency low.